As outlined in Healthy People 2010, eliminating health disparities, especially with respect to child obesity, is a major US goal. In the LA Unified School District (LAUSD), the community partner for the proposed research, 43% of elementary and secondary school children in the district are obese or overweight, with the highest prevalence among Latinos (45%) and Blacks (41%), compared to Whites (29%) and Asians/Pacific Islanders (27%). Limited physical activity and poor diet contribute to obesity among youth from all racial/ethnic groups, especially Blacks and Latinos. We propose a 5-year randomized controlled trial of a school-based obesity prevention intervention to address racial/ethnic disparities in nutrition and physical activity among middle school youth in LAUSD. We aim to: (1) determine whether a school-based community-based participatory research intervention can increase healthy eating, physical activity, and physical fitness, and decrease sedentary behavior; (2) examine racial and ethnic differences in the effectiveness of the intervention; (3) assess whether theoretically important correlates of behavioral change (i.e., attitudes, social norms, and self-efficacy) predict changes in the behaviors targeted by the intervention; (4) estimate the costs and benefits of the intervention and calculate cost-effectiveness ratios; and (5) disseminate intervention results to community stakeholders and develop a plan for broad program dissemination. In a Planning Grant conducted under NIH/NCMHD RFA-MD-05-002 (R24 MD001648; MA Schuster, PI), we established new relationships and strengthened existing relationships among community partners at LAUSD, key stakeholders invested in youth and obesity prevention from across LA County, and researchers at the Center for Adolescent Health Promotion. This partnership has become a strong and thriving collaboration that has led to our developing, pilot testing, and refining our new school-based intervention, Students for Nutrition and exercise (SNaX). SNaX combines improvements to the school food environment and school-wide multimedia promotions for 7th graders and their parents with an after-school club in which peer leaders are encouraged to use a motivational interviewing style to promote healthy eating and physical activity to family and peers. SNaX aims to reduce racial/ethnic disparities in healthy eating and physical activity by empowering adolescents to be advocates for change within their communities.